Relationship help.. Too much overthinking?

I'm currently in a relationship with someone i met through a friend. He is genuine, funny, an all round great person. And i am happy being with him. I am 18, just finished college and now get to see a lot more of him. We're 4 months into the relationship - and things progressed very quickly to start with. Despite how great it's been so far, i can't help but doubt and question things he says. For example, when he talks about how he feels towards me, when he says 'im different from other girls he's been with' - i can't help but be sceptical and choose not to believe it. He's hinted at saying the big 'L' word - and i know he's said that to 'at least 3' of his exs - so how am i meant to know i'm any different? I once (very wrongly) had a peek on his facebook messages with a girl who is his ex.. The things i read didn't make me feel so 'special' anymore. The things i thought we did together, which were 'our thing' turned out to be what he had done with her also. It wasn't just one or two things, there were quite a few and it just increased my doubts and scepticism. Little things like, giving my his old dog tags, which i found out he had done for another girl. Which yeah, it's sweet and maybe his thing, but i felt so special when he said i could have them. (that may seem really petty to some one reading this, i just read it back over.. haha) I feel like he doesn't ask me about my life - i've had a bit of a ****ty past (who hasn't?) and i know he doesn't want to know about my past relationships, but he doesn't seem to question or be curious regarding other areas of my life. If i hadn't offered him the information, he would be none the wiser. Whereas i find myself asking him about his life quite a lot in comparison I think this is a really bad mix of paranoia and over thinking on my behalf. And possibly this may seem kind of ridiculous to some people reading this... :$ but i just don't know what to do/think regarding the relationship. What are your views? Had anything similar happen to you? (if you have read this, thankyou!) :)

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